Electro mobils are only economical in a quite narrow loading range if the power transmission is provided with a rigid step down ratio. Although electronic control systems may bring an amelioration of its commerciality the driving power required has to be quite high for overcoming only small slopes. Therefore, gearboxes have to be provided to enable altering the transmission ratio in steps. Hydraulic gears, especially used for transmission of small powers show too small efficiencies. Vario-couplers which work as torque converter are primarly known based on hydrostate technique and are very expensive.